1. Field of the Invention
The present invention relates to a radar, and more particularly, to a radar which emits a transmission wave, such as a radio wave or light, to a target, receives a wave reflected from the target, and computes a distance between the radar and the target from a time difference.
2. Description of the Related Art
FIG. 13 is a view for describing operation of a typical radar in the related art. On the occasion of detection of a distance from the position of a radar to a target; e.g., a vehicle such as an automobile, a voltage level is obtained along with a conversion characteristic which varies in the form of a straight line within a range from L3 to L1 and is indicated by a line 1 shown in FIG. 13, provided that L3 designates a reflection limit distance which can be detected by the radar; e.g., 60 m, and that L1 designates a minimum detection distance L1; e.g., 0.5 m. The voltage obtained at the detectable distance L3 is Vmax, and a voltage obtained in accordance with the minimum detection distance L1 is Vmin. A voltage ΔV1 is a voltage corresponding to a delay time pertaining to the operation of an electric circuit from the time a transmission wave is emitted until a received wave is output. The voltage obtained within the range from Vmin to Vmax is subjected to analog-to-digital conversion (abbreviated as A/D), thereby detecting a distance.
FIG. 14 is a view showing a conversion characteristic of another radar in the related art. The related art shown in FIG. 14 is analogous that shown in FIG. 13. According to the related art shown in FIG. 13, a corresponding voltage increases with an increase in the distance to the target. However, the related art shown in FIG. 14 has a conversion characteristic such that a corresponding voltage decreases from the maximum voltage Vmax that has been decreased by the voltage ΔV1 as the distance to the target becomes greater.
In any of the related-art techniques shown in FIGS. 13 and 14, the voltage Vmin to Vmax corresponding to the time difference is expressed as a linear function of distance over a comparatively wide range of distance from the minimum detection distance L1 to the reflection limit distance L3. Accordingly, there arises a problem of low resolving power within the range of distance from the minimum detection distance L1, which is most frequently required, to a detection-necessary distance L2 (e.g., 10 to 20 m), which is a required distance; e.g., a distance required at the time of controlling of the vehicle's brake. In these related-art techniques, the voltage—which is obtained in accordance with the time difference and represented by the longitudinal axis shown in FIGS. 13 and 14—includes the voltage ΔV1 corresponding to a delay period pertaining to the operation of the electric circuit. This also results in narrowing of the range of the voltage Vmin to Vmax used for detecting a distance, and consequently the resolving power of a distance falls further.
The related-art techniques serving as the background of the present invention are described in JP-UM-A-5-28983.